Polyimides have widespread industrial use. In particular, polyimides are commonly employed in the semiconductor and packaging industry. Polyimides are usually employed for metal passivation or conductor insulation, particularly, because the polyimides exhibit low dielectric characteristics along with high thermal and chemical stability.
In packaging of semiconductor chips, polyimide films are often coated onto substrates. Typically, a polyamic acid or alkyl ester precursor of the polyimide is applied by spin coating onto the desired substrate, and subsequently cured by thermal excursions of up to about 400.degree. C. However, one problem associated with such a process is that the polyimide precursors employed are reactive with metals such as copper. This in turn causes oxidation of the metal which leads to the incorporation of metal oxide into the polymer bulk during the curing cycle. The presence of the metal oxide adversely affects the dielectric properties of the polyimide and the reliability of the metal-polyimide interface. Another problem associated with applying the polyamic acid or polyamic ester precursor is that the heating results in imidization, i.e., ring closure with concurrent release of water (for polyamic acids) or alcohol (for polyamic esters).
This curing process results in weight loss and dimensional changes in the polymer such as shrinking. This concern can be minimized by applying a preimidized polyimide coating. However, most polyimides, especially those possessing the best packaging properties, are not soluble and therefore cannot be applied in the imidized form.
A polyimide has an imide group which has a variety of isomers. We have discovered that by supplying an electron to an isoimide to form a reduced isoimide it is energetically disposed toward transformation to an n-imide. It is believed that the originally supplied electron remains on the n-imide as a reduced imide which permits electrochemical deposition of the reduced n-imide onto an electrically conducting substrate.
We have additionally discovered that the n-imide can use the initially supplied electron for initiating the isomerization of another isoimide molecule to an n-imide molecule. Therefore, the originally supplied electron is not lost but remains in the system. The isoimide transformation to the n-imide is catalytic to the supplied electron which remains in the system.
A polyisoimide is generally more soluble in solvents than is poly-n-imide which is often insoluble. Therefore, starting with a solution of polyisoimide which, when reduced, is energetically disposed to isomerize to a poly-n-imide which readily deposits into an electrically biased conductor, is a substantially more efficient process than starting with a solution of reduced poly-n-imide. The enhanced efficiency comes from the enhanced isoimide solubility and the catalytic isoimide to n-imide transformation which re-supplies the electron for use in additional transformations. In this way, more rapid deposition can be achieved. The original electron is preferably supplied from an electrode in solution to form a reduced isoimide when starting with an isoimide and to form a reduced imide when starting with an n-imide. In the former situation the electron is available for re-use in another transformation where as in the latter situation the electron is not available for reducing another poly-n-imide.
It is an object of this invention to transform an isoimide to an n-imide by supplying an electron to an isoimide which is then energetically disposed toward transformation to the n-imide.
It is another object of this invention to electrolytically deposit an imide compound onto a conducting surface starting with an isoimide.